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Tectonophysics 410 (
Crustal constraints on the origin of mantle seismicity in the
Vrancea Zone, Romania: The case for active continental
lithospheric delamination
James H. Knapp a,*, Camelia C. Knapp a, Victor Raileanu b, Liviu Matenco c,
Victor Mocanu c, Cornel Dinu c
a Department of Geological Sciences University of South Carolina Columbia, SC 29208, United Statesb National Institute of Earth Physics, Magurele, Bucharest, Romania
c Faculty of Geology and Geophysics, University of Bucharest, Bucharest, Romania
Received 15 April 2004; received in revised form 30 September 2004; accepted 10 February 2005
Available online 17 November 2005
Abstract
The Vrancea zone of Romania constitutes one of the most active seismic zones in Europe, where intermediate-depth (70–200
km) earthquakes of magnitude in excess of Mw=7.0 occur with relative frequency in a geographically restricted area within the
1108 bend region of the southeastern Carpathian orogen. Geologically, the Vrancea zone is characterized by (a) a laterally
restricted, steeply NW-dipping seismogenic volume (30�70�200 km), situated beneath (b) thickened continental crust within the
highly arcuate bend region of the Carpathian orocline, and (c) miscorrelation of hypocenters with the position of known or inferred
suture zones in the Carpathian orogenic system. Geologic data from petroleum exploration in the Eastern Carpathians, published
palinspastic reconstructions, and reprocessing of industry seismic data from the Carpathian foreland indicate that (1) crust of
continental affinity extends significantly westward beneath the external thrust nappes (Sub-Carpathian, Marginal Folds, and
Tarcau) of the Eastern Carpathians, (2) Cretaceous to Miocene strata of continental affinity can be reconstructed westward to a
position now occupied by the Transylvanian basin, and (3) geologic structure in the Carpathian foreland (including the Moho) is
sub-horizontal directly to the east and above the Vrancea seismogenic zone. Taken together, these geologic relationships imply that
the Vrancea zone occupies a region overlain by continental crust and upper mantle, and does not appear to originate from a
subducted oceanic slab along the length of the Carpathian orogen. Accordingly, the Vrancea zone appears to potentially be an
important place to establish evidence for active lithospheric delamination.
D 2005 Published by Elsevier B.V.
Keywords: Delamination; Vrancea zone; Romania; Tectonics; Carpathians
1. Introduction
Ever since the recognition of deep focus earthquakes
(Wadati, 1928), the cause of mantle seismicity has been
0040-1951/$ - see front matter D 2005 Published by Elsevier B.V.
doi:10.1016/j.tecto.2005.02.020
* Corresponding author. Tel.: +1 803 777 6886; fax: +1 803 777
6082.
E-mail address: [email protected] (J.H. Knapp).
a long-standing problem in the Earth sciences. While
early workers observed that earthquake focal depths
generally increase with distance from the trench, form-
ing dipping seismogenic planes in the mantle (Wadati,
1928, 1935; Benioff, 1949), it was many years before
such seismicity was correlated with the subduction of
plates of oceanic lithosphere. Development of plate-
tectonic theory in the late 1960s subsequently provided
2005) 311–323
J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323312
a fundamental new framework in which to explain the
origin and geodynamic significance of earthquakes
occurring at depths of 70–700 km, along so-called
Wadati–Benioff zones (Isacks et al., 1968). We now
commonly understand such intermediate-to deep-focus
seismicity as brittle deformation of descending, rigid
oceanic slabs at subduction zones.
In the past two decades, delamination of thickened
continental lithosphere has gained favor as an important
geodynamic process in orogenic settings (e.g., Nelson,
1991), and has been proposed as an alternative mech-
anism for generation of mantle seismicity (e.g., Seber et
al., 1996). Such an interpretation seems plausible where
earthquake foci do not align along a dipping Wadati–
Benioff plane, but cluster in a more concentrated vol-
ume. dDelaminationT was first introduced in the late
1970s within a theoretical context (Bird, 1978, 1979),
and was originally conceived as detachment of thick-
ened lithospheric mantle from overlying crust during
continental collision (Bird, 1978). Subsequently, the
concept has come to encompass a much broader
range of processes, including removal of material
from the base of the lithosphere due to gravitational
instability (Bird, 1979), detachment of oceanic slabs
(Sacks and Secor, 1990), or foundering of mafic
lower crust and upper mantle driven by phase changes
(Kay and Kay, 1993; Nelson, 1991).
Regardless of the specific mechanism, the removal
of continental lithosphere into the underlying mantle
has been routinely invoked to explain the geologic
evolution of orogenic systems (e.g., Furlong and Foun-
tain, 1986; Dewey, 1988; Austrheim, 1991; Baird et
al., 1996). If such a process has operated regularly in
the development of mountain belts, it has far-reaching
implications for the evolution of continental litho-
sphere (e.g., Nelson, 1991), the dynamics of the
crust/mantle (Moho) and lithosphere/asthenosphere
boundaries, and perhaps most significantly, the long-
term geochemical budget of both crust and mantle. The
tectonic and geodynamic consequences of lithospheric
delamination are generally agreed to include (1) post-
orogenic extensional collapse, (2) regional uplift, (3)
deep-seated alkaline magmatism, (4) elevated heat
flow, and (5) subcrustal seismicity. Accordingly, most
workers have looked to the geologic record to docu-
ment evidence for these collective phenomena in the
geologic past, however, unequivocal proof for a mod-
ern example of lithospheric delamination has yet to be
documented. Such evidence could presumably only
come from a setting where mantle earthquakes are
now occurring in the demonstrated absence of a sub-
ducted slab.
The Vrancea zone of Romania constitutes one of the
most active seismic zones in Europe, where mantle
(70–200 km) earthquakes of magnitude in excess of
Mw=7.0 occur with relative frequency in a geograph-
ically restricted area (Fig. 1). For centuries, these seis-
mic events have resulted in a high toll of human
casualties and property damage, and make Bucharest
today one of the largest population centers in Europe at
significant seismic risk. Geologically, the Vrancea zone
is characterized by (a) a laterally restricted, steeply NW-
dipping seismogenic volume (30�70�200 km; Wen-
zel et al., 1998), situated beneath (b) thickened conti-
nental crust within the highly arcuate bend region of the
Carpathian orocline (Radulescu, 1981), (c) miscorrela-
tion of hypocenters with either documented or inferred
suture zones in the Carpathian orogenic system (Linzer,
1996), and (d) pronounced and localized Late Miocene/
Pliocene subsidence in the Focsani basin of the fore-
land (Artyushkov et al., 1996; Matenco et al., 2003), an
area that also exhibits (e) active surface deformation
and crustal seismicity. These geologic relationships are
for the most part specific to the bend region, and as
such, do not appear to support the premise of a sub-
ducted oceanic slab along the length of the Carpathian
orogen. Accordingly, the Vrancea zone may be the
unique place to establish evidence for active lithospher-
ic delamination.
Despite these considerations, and several recent seis-
mological investigations focused on the mantle source
region of the Vrancea zone (Fan et al., 1998; Wenzel et
al., 1998; Wortel and Spakman, 2000; Hauser et al.,
2001), the weight of scientific opinion remains over-
whelmingly in favor of a subducted slab origin for
Vrancea seismicity. Both active- and passive-source
studies in Romania in the last decade have resulted in a
substantially improved understanding of the velocity
structure of the crust and upper mantle. Similarly, the
spatial relationship of Vrancea mantle seismicity to ac-
tive surface deformation in the foreland suggests that
these regions are mechanically coupled through the crust
and upper mantle, which would be a fundamental con-
straint on the competing geodynamic models.
Here we present interpretations of existing data from
petroleum exploration and reprocessing of deep seismic
reflection data that document the continuity of conti-
nental crust beneath the external nappes of the Eastern
Carpathians, and accordingly, above the Vrancea seis-
mic zone. These surface and subsurface data appear to
preclude the possibility that a slab, either still attached
or now detached, was subducted either in place within
the Carpathian foreland (e.g., Wortel and Spakman,
2000) or beneath the Eastern Carpathians (e.g., Wenzel
Fig. 1. Color-shaded DEM of Romania, emphasizing the highly arcuate nature of the Carpathian orogen, and location of intermediate-depth
seismicity of the Vrancea zone at bend joining Eastern and Southern Carpathians. Focal mechanisms of earthquakes (MwN5.0, 1977–2001) from the
CMT catalogue (Harvard, USA) displayed as beach balls. Crustal seismicity (mbN4) from the ISC catalogue displayed as black dots. Note the
elevated topography of the hinterland (Transylvanian) and foreland basins of the Eastern Carpathians, and Quaternary extensional basins within the
interior of the bend region. Topographic data from USGS GTOPO 30 (Smith and Sandwell, 1997).
J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323 313
et al., 1998; Girbacea and Frisch, 1998; Gvirtzman,
2002). Accordingly, the NW-dipping seismogenic
body of the Vrancea zone may more likely have orig-
inated as delaminating continental lithosphere (Diaco-
nescu et al., 2000, 2001; Knapp et al., 2001), or through
some as yet unidentified geodynamic process for gen-
erating spatially restricted intermediate-depth seismicity
in the absence of subducted oceanic lithosphere.
2. Geologic background
Formation of the Carpathian orogen (Figs. 1 and 2)
can be broadly understood in the framework of Meso-
zoic and Cenozoic closure of the Tethys ocean during
continental collision of the Eurasian and African plates.
Two main periods of compressional deformation are
recognized in the Eastern Carpathians, one during
Late Cretaceous time that was responsible for emplace-
ment of large crystalline thrust sheets (Getic, Supra-
getic, and Bucovinian nappes) now exposed in the
westernmost Eastern Carpathians, and a second phase
during Early and Middle Miocene time that involved
imbrication of a Cretaceous through Miocene strati-
graphic sequence in the external nappes (Sandulescu,
1988; Fig. 2). This sedimentary section is tectonically
detached from the basement upon which it was depos-
ited, and records Miocene-age thin-skinned shortening
of as much as 180 km (Roure et al., 1993; Ellouz et al.,
1994). Neogene strata within the external nappes show
a clear stratigraphic affinity with the East European and
Moesian continental basement on which they now rest
tectonically (e.g., Sandulescu, 1980). Final nappe em-
placement in the Eastern Carpathians was mid-Miocene
(11–9 Ma) in age, and was followed by continued
compression and backthrusting in Pliocene time (Stefa-
nescu, 1986; Sandulescu, 1988; Horvath and Cloetingh,
1996; Matenco, 1997; Matenco and Bertotti, 2000;
Ciulavu et al., 2000).
Tectonic elements of the Carpathian system that are
critical to the evaluation of competing geodynamic
models for the origin of Vrancea seismicity are high-
lighted in Fig. 2 and include (1) the Cretaceous (Trans-
ylvanide) suture zone of the hinterland, (2) the crustal
structure of the Transylvanian basin, (3) an enigmatic
Neogene volcanic arc developed behind and on top of
the fold and thrust belt, (4) foreland subsidence in the
Focsani basin, (5) basement structure in the foreland as
it pertains to active deformation there, and (6) Quater-
nary basins which are forming within the bend region
of the Carpathian fold belt and hinterland.
Fig. 2. Geologic map of Romania (modified after Sandulescu et al., 1978), emphasizing (1) position of mid-Cretaceous suture zone (Transylvanides) in the Transylvanian basin, (2) late Tertiary
volcanic arc (red) and (3) thrust nappes of Eastern Carpathians developed in Cretaceous to Miocene strata (green, orange, and yellow units) deposited on the East European/Moesian continental
plates. Thrust deformation in the Eastern Carpathians was primarily Early to Middle Miocene in age. Locations of geologic sections in Figs. 3(Section 1) and 4(Section 2) indicated by heavy black
lines. Sections A and B are deep seismic reflection profiles projected into the cross-section X–XV (light grey) in Fig. 5.
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J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323 315
Burchfiel (1976) and Sandulescu (1988) first sug-
gested that a marginal ocean basin once occupied the
area of the present-day Carpathian/Pannonian system.
Evidence for closure of an ocean basin during Creta-
ceous time is preserved in ophiolites (Transylvanides)
between the Southern Carpathians and northern Apu-
seni Mountains (Sandulescu, 1988; Royden and Baldi,
1988; Fig. 2). Using gravity and magnetic data, this
ophiolitic complex was traced toward the NE beneath
the Transylvanian basin and was interpreted to be the
root zone of a Transylvanian nappe (Fig. 2; Sandulescu,
1980).
The case for closure of an ocean-floored basin in
Miocene time is far less compelling. Scattered occur-
rences of Mesozoic mafic rocks occur along the inner
(western) margin of the Eastern Carpathians, structur-
ally above the crystalline basement rocks of the Getic
and Supragetic nappes of the Eastern Carpathians (Fig.
2). Many studies have interpreted these mafic rocks as
evidence for subduction of oceanic lithosphere during
Miocene formation of the Eastern Carpathians (Balla,
1987; Sandulescu, 1988; Csontos et al., 1992; Csontos,
1995; Ratschbacher et al., 1993; Linzer, 1996). In turn,
it is this putative Miocene slab that many workers now
interpret as the source for Vrancea seismicity (e.g.,
Linzer, 1996; Wortel and Spakman, 2000). It remains
unclear whether these rocks of oceanic affinity are
rooted in the crust, or whether the eastern Transylva-
nian basin is underlain, at least in part, by continental
crust of East Europe and Moesia.
Probably one of the strongest arguments for subduc-
tion of an oceanic slab beneath the Eastern Carpathians
is the presence of a linear arc of Neogene volcanism
within the hinterland (Fig. 2). This volcanic chain,
comprised of both calc-alkaline and alkaline magmas,
was active from Middle Miocene to Quaternary time
(13.4–0.2 Ma), and migrated successively from north to
south (Mason et al., 1998). Although major- and trace-
element geochemistry of the calc-alkaline lavas suggest
they are subduction-related (e.g., Pecskay et al., 1995;
Mason et al., 1998), (1) this magmatic activity largely
post-dated the final stages of deformation in the Eastern
Carpathians, and (2) the volcanic chain actually cross-
cuts the surface trace of the putative Miocene subduc-
tion zone from which it was presumably derived (Fig.
2). Salters et al. (1988) offered an alternative suggestion
that both calc-alkaline and alkaline lavas of the Car-
pathian hinterland could be derived from a single man-
tle source, and reflect differing degrees of crustal
contamination.
Several hypothetical models for the geodynamic
setting of the Vrancea zone have been posed, revolving
primarily around variations on subduction of oceanic
lithosphere. Numerous authors (e.g., Balla, 1987; Cson-
tos et al., 1992) have suggested that oceanic lithosphere
attached to the East European craton (Fig. 1) was
subducted west- and southwestward along the entire
Carpathian arc during Miocene time, and now coincides
with the Vrancea zone. In this model, the subducting
slab presumably began to progressively tear once thick
continental crust entered the subduction zone at ~70 km
depth (e.g., Wortel and Spakman, 2000). Alternatively,
some authors (Linzer, 1996; Girbacea and Frisch, 1998;
Mason et al., 1998) have proposed that the subducting
basin was attached to the Moesian platform (Fig. 1),
and was subducted northwestward beneath the Car-
pathian orogen. Subsequent large scale roll-back from
NW to SE over a distance of ~130 km resulted in
positioning of this detached slab with the Vrancea
zone. Such a model is in agreement with (1) migration
and diminution of Neogene calc-alkaline magmatism in
the Eastern Carpathians from northwest to southeast
along the arc (Mason et al., 1998), and (2) the NW-
dipping geometry of the Vrancea seismic body (Girba-
cea and Frisch, 1998).
3. Discussion
Existing geological and geophysical data from the
Eastern Carpathians and the adjacent foreland place
critical constraints on the geodynamic setting of the
Vrancea zone. Both surface and subsurface data along
the length of the Eastern Carpathians argue that conti-
nental crust of the East European and/or Moesian plat-
forms can be traced, as a minimum, beneath the
external nappes, and above the locus of Vrancea seis-
micity. On this basis, any model explaining the origin
of Vrancea seismicity must account for the existence of
this seismogenic zone beneath continental lithosphere.
Shown in Fig. 3 is a geologic section through the
frontal nappes of the Eastern Carpathians based on
petroleum industry data (Dicea, 1996). Deep wells
from this transect in the Moldova River valley (see
Fig. 2 for location) penetrate deformed strata associated
with the Tarcau, Marginal Folds, and Sub-Carpathian
nappes, before sampling Cenozoic (Sarmatian and
Badenian) and Mesozoic formations of the autochtho-
nous cover of the East European platform. While the
westernmost well on this section is situated more than
10 km west from the exposed thrust front of the Eastern
Carpathians, the Mesozoic and underlying Paleozoic
platform strata at depth appear to project considerably
further to the west. Based on such data, it follows that
the autochthonous continental basement on which these
Fig. 3. Geologic cross-section (WSW–ENE) through the external nappes of the northern Eastern Carpathians, based on control from petroleum
exploration wells (modified after Dicea, 1996). Location of wells shown by vertical lines. Major thrust faults shown by heavy lines at base of thrust
nappes. Medium weight lines indicate faults; lighter lines indicate formational contacts. Legend (below) shows stacking order of nappes, emplaced
from west to east. Note that the projection of the VSZ along structural strike of the Eastern Carpathians clearly corresponds to a position overlain by
continental crust. See Fig. 2(Section 1) for location.
J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323316
strata were deposited extends a considerable distance
westward beneath the external nappes of the Car-
pathians. Minimum figures reported by Dicea (1996)
are 20 km in the Moldova Valley, and 30 km in the
Bistrita and Trotus valleys further to the south.
Unfortunately, such seismic and well data sampling
the rocks beneath the external nappes near the Vrancea
zone are not currently available. Given the highly cy-
lindrical structure of the Eastern Carpathians, however,
projection of this crustal configuration southward along
orogenic strike seems reasonable. While this projection
is admittedly more than 200 km removed from the
Vrancea zone, the surface geology exposed within the
nappes of the Eastern Carpathians is remarkably later-
ally consistent, and implies a continuity of geologic
structure since Mesozoic time. Similarly, the stratigra-
phy of the Cretaceous to Miocene stratigraphic section
exposed in the Eastern Carpathians is quite continuous
along strike (Contescu, 1974). Faults such as the Trotus
fault have been interpreted as major crustal boundaries
within the Carpathian foreland (Matenco, 1997; Tara-
poanca et al., 2003), but such structures are not mani-
fest in either the stratigraphy or structure of the external
Carpathian nappes (Fig. 2), implying that the basement
upon which these strata were deposited and subsequent-
ly emplaced behaved uniformly along strike. As shown
in Fig. 3, projection of Dicea’s (1996) section, con-
trolled by well penetrations of the autochthonous base-
ment of the East European platform, implies continental
crust lies both updip of and above the steeply northwest
dipping Vrancea zone.
In the central portion of the Eastern Carpathians,
Burchfiel (1976) produced a palinspastic reconstruction
of deformation, extending from the autochthonous plat-
form in the east to the crystalline thrust nappes (Bucov-
inian) exposed in the hinterland (Fig. 4). While no one
thrust nappe contains the entire Cretaceous to Tertiary
sequence, a contiguous stratigraphic section from Late
Cretaceous through Pliocene age can be reconstructed
across the successive nappes of the Eastern Carpathians.
Of critical importance are the observations that (1) this
stratigraphic section can be tied to Neogene strata that
were clearly deposited on the autochthonous continental
platform, and (2) such stratigraphic continuity across the
Eastern Carpathians appears to preclude a significant
intervening basin floored by oceanic crust during Neo-
gene time. Published seismic reflection sections at this
same latitude (Dicea, 1995) image the platform stratig-
raphy extending a minimum of 10 km (and presumably
much further) westward beneath the frontal edge of the
Sub-Carpathian nappe. Furthermore, Burchfiel’s (1976)
section results in an estimate of ~125 km of shortening
Fig. 4. Palinspastic reconstruction (below) through central portion of the Eastern Carpathians (modified after Burchfiel, 1976), based on W–E structural cross-section (above). Reconstruction is
pinned at eastern end, within the autochthonous crust of the East European craton. A contiguous Cretaceous to Pliocene stratigraphic section can be traced from the autochthon to the western end of
the section (crystalline Bucovinian nappes) without interruption. See Fig. 2(Section 2) for location.
J.H.Knappet
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J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323318
within the central Eastern Carpathians, implying that at
the latitude of this section, crust of continental affinity
extends a similar distance westward beneath the Eastern
Carpathian nappes.While the rocks of the Ceahlau nappe
may be an exception, it is certainly plausible on the basis
of this analysis that East European and/or Moesian con-
tinental crust projects tens of kilometers west of the
current known extent, and did so prior to shortening in
Miocene time.
As with the exposed stratigraphy and shallow geo-
logic structure of the Eastern Carpathians, the crustal-
scale structure of the Carpathian foreland provides
additional constraints on the origin of Vrancea seismic-
ity. Deep seismic reflection data from the Carpathian
foreland (Ramnicu Sarat profile) and the Transylvanian
hinterland (Targu Mures I profile) are combined in a
lithosphere-scale cross section across the Vrancea zone
(Fig. 5; see Fig. 2 for location). While some dipping
structure is evident at the basement-cover contact with-
in the deep Focsani basin, crustal fabrics and structure
at the Moho are comparatively sub-horizontal, and
inconsistent with a significant bending of the crust
into the Vrancea seismic zone. Indeed, strata presum-
ably deposited on continental crust (based on palinspas-
Fig. 5. WNW–ESE transect (~320 km) along cross-section X–XV showing i
dots). Topography across the profile (above) shown in m, and depth below se
reprocessed (Ramnicu Sarat) versions of deep profiles (A and B in Fig. 2; se
from refraction and gravity modeling (Radulescu, 1981).
tic reconstruction of the fold and thrust belt) occupy the
high topography of the Eastern Carpathian directly
above the Vrancea zone. While it may yet be permis-
sible that the continental crust imaged by these deep
seismic data in the foreland rolls over essentially verti-
cally into the adjacent Vrancea zone (Fig. 5), such a
geometry does not appear consistent with geologic data
presented here.
Based on these geological and geophysical observa-
tions, three contrasting models have been or can be
posed for the geodynamic setting of the Vrancea region
(Fig. 6): (A) oceanic slab break-off and retreat, (B)
oceanic slab subduction within the foreland, and pro-
gressive lateral tearing of the slab, or (C) lithospheric
delamination due to continental underthrusting and oro-
genic thickening. Each of these envisions a relatively
cold and dense body of lithospheric material that is
presently situated in the upper mantle beneath the
bend region generating seismicity in the Vrancea re-
gion. The first two models require subduction of oce-
anic lithosphere to explain Vrancea seismicity, and can
be distinguished on the basis of where the proposed
subduction occurred within the orogen, and how the
subducted slab evolved (break-off and trenchward re-
nterpreted crustal structure above Vrancea zone (hypocenters in black
a level (below) shown in km. Insets show industry (Targu Mures I) and
ctions plotted ~1:1 at 6 km/s). Moho beneath Carpathians determined
Fig. 6. 3-D perspective lithosphere-scale block models (view towards NNW), illustrating contrasting scenarios for geodynamic setting of the Vrancea zone. (A) Oceanic slab subduction and break-
off. (B) Oceanic slab subduction and progressive lateral tear within the Carpathian foreland. (C) Continental lithospheric delamination. Green = Moesian/East European crust; yellow =
Transylvanian crust; pink = continental mantle lithosphere; purple = oceanic lithosphere; grey = asthenosphere. Vrancea zone is located in lower front corner of models. See text for discussion.
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J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323320
treat vs. migration of a lateral tear along the orogen).
The third model could be generated through closure of
an intracontinental basin and lithospheric thickening,
and does not require the former presence of an ocean-
floored basin, or the subduction of oceanic lithosphere.
Each of these models has implications for the asso-
ciated crustal structure, and is readily testable through
integration of surface and subsurface observations. In
particular, if an ocean-floored basin was consumed
during the formation of the Eastern Carpathians, evi-
dence for the former boundary between two distinct
continental plates should be recorded in both the sur-
face geology (suture zone) and the underlying crustal
structure. The predicted position of such a boundary
differs by more than 150 km in the two subduction
models (Figs. 5 and 6A and B). To date, no conclusive
evidence has been documented for such a crustal (and
lithospheric) boundary in either position. Furthermore,
the tectonic affinity of Neogene strata exposed in the
Eastern Carpathians should be diagnostic. Models A
(slab break-off) and C (continental delamination) clear-
ly imply an East European/Moesian crustal affinity for
these Neogene age strata, whereas model B (slab tear
model) requires that these strata were separated from
the East European/Moesian margin by an intervening
ocean-floored basin.
Although widely accepted in the scientific literature,
the subduction model for the Vrancea zone presents
several clear weaknesses. The Vrancea zone, if now
represented by a remnant slab, does not coincide updip
with the surface expression of a suture zone. Similarly,
Neogene volcanism west of the Eastern Carpathians
occurs in such close proximity (b50 km) to the zone of
modern seismicity (Fig. 2) that any subducting slab
would not likely be at depths to dewater and induce
melting of the overlying mantle. Vrancea zone earth-
quakes are located beneath the external fold and thrust
belt and the adjacent Focsani depression. Furthermore,
eruption of Quaternary alkaline basalts in eastern
Transylvania appears to be unrelated to subduction
volcanism. Mason et al. (1998) appealed to late-stage
slab break-off and asthenospheric upwelling to gener-
ate these basalts. Perhaps most importantly, if such a
detached slab is present, it did not sink vertically
through the mantle, but moved by ~130 km horizon-
tally to the southeast relative to the suture zone iden-
tified in Transylvania. Moreover, development of
extensional sedimentary basins of Miocene–Quaternary
age in the vicinity of the Vrancea zone, as well as
extensive development of uplifted fluvial terraces in
the foreland, would appear to be at odds with subduc-
tion geodynamics.
While continental delamination was proposed as a
mechanism to produce mantle seismicity in the Alboran
Sea (Seber et al., 1996; Mezcua and Rueda, 1997),
Vrancea zone seismicity has not been widely viewed
in this light. Some workers (Csontos, 1995; Girbacea
and Frisch, 1998; Gvirtzman, 2002) have appealed to
foundering of lithospheric mantle as the final stages of
slab subduction beneath the Eastern Carpathians, but
these models still require the consumption of oceanic
lithosphere in Miocene time. A number of observations
appear consistent with active delamination of continen-
tal mantle lithosphere in the southeastern Carpathians
including: (1) the narrow, cylindrical shape of the seis-
mogenic zone, and implicitly, the lack of a well-defined
Benioff plane, (2) the miscorrelation of hypocenters
(spatially) and volcanism (both spatially and temporal-
ly) with the expected position of a bnormalQ descendingslab (Girbacea and Frisch, 1998), (3) the eruption of
Quaternary alkali basalts in the vicinity of the Vrancea
zone, (4) the presence of an aseismic gap between 40
and 70 km depth, with low P-wave velocity (Fuchs et
al., 1979; Oncescu, 1984; Lorenz et al., 1997; Fan et al.,
1998), and high attenuation (Q; Mocanu et al., 1999),
(5) a weak zone beneath the crust based on lithosphere
strength modeling (Lankreijer et al., 1997), (6) the late
Miocene to Pleistocene subsidence of the Carpathian
foreland basin in the absence of major surface loads
(Artyushkov et al., 1996), (7) the development of Qua-
ternary extensional basins in the vicinity of the Vrancea
zone (Ciulavu et al., 2000), and (8) active deformation
in the Carpathian foreland, concentric about the locus
of Vrancea zone seismicity, and recording displacement
rates of up to 15 mm/yr.
To a first order, the presence or absence of a
through-going, dipping crustal fabric beneath the pur-
ported Miocene suture would alternatively substantiate
or eliminate the subduction-model origin for Vrancea
seismicity. Similarly, demonstration of a geometric as-
sociation of foreland deformation with the Vrancea
mantle source region would imply a mechanical cou-
pling of the Vrancea seismogenic body with the over-
lying crust, and hence argue against a detached slab.
Deep seismic reflection data are uniquely suited to
address these competing hypotheses by providing a
high-resolution image of the structural geometry of
the crust, and a direct link of surface geology to
upper mantle seismicity.
4. Conclusions
The origin of intermediate depth seismicity in the
Vrancea zone of Romania continues to be a subject of
J.H. Knapp et al. / Tectonophysics 410 (2005) 311–323 321
debate. While most workers consider that the Vrancea
seismogenic body consists of oceanic lithosphere, ei-
ther resulting from detachment and lateral migration of
an oceanic slab (e.g., Girbacea and Frisch, 1998;
Gvirtzman, 2002), or subduction and lateral tearing of
a slab beneath the eastern edge of the Eastern Car-
pathians (e.g., Wenzel et al., 1998; Wortel and Spak-
man, 2000), such interpretations appear to be
inconsistent with geologic constraints from the Eastern
Carpathians and adjacent foreland. In particular, petro-
leum exploration data document that crust of continen-
tal affinity extends significantly westward beneath the
external thrust nappes (Sub-Carpathian, Marginal
Folds, and Tarcau) of the Eastern Carpathians. In addi-
tion, Neogene strata of the Eastern Carpathians can be
reconstructed much further westward to a position now
occupied by the Transylvanian basin. Finally, geologic
structure in the Carpathian foreland (including the
Moho) is sub-horizontal directly to the east of and
above the Vrancea seismogenic zone. Taken together,
these geologic relationships imply that the Vrancea
zone occupies a region overlain by continental crust
and upper mantle, and does not appear to originate from
a subducted oceanic slab along the length of the Car-
pathian orogen.
An alternative model for Vrancea zone seismicity,
proposed here, involves active continental lithospheric
delamination, resulting from Miocene closure of an
intra-continental basin and attendant lithospheric thick-
ening. Such a model is consistent with the presence of
seismicity beneath thick continental crust, including the
foreland of the Carpathian system, and honors surface
and subsurface geologic constraints that appear to pre-
clude the presence of remnant oceanic crust in Tertiary
time in the region.
Acknowledgements
We have benefited greatly from discussions with M.
Sandulescu, R. Russo, and G. Yogodzinski, as well as
insightful comments by J. McBride and L. Csontos,
and an anonymous reviewer, however we remain re-
sponsible for any misstatements or errors of fact. This
work was supported by an award to JHK from the
National Research Council COBASE program, and
award EAR-0310118 to JHK and CCK from the Tec-
tonics Program of the National Science Foundation.
CCK acknowledges the donors of The Petroleum Re-
search Fund, administered by the ACS, for partial
support of this research. Seismic data processing was
facilitated through a grant from the Landmark Graphics
Corporation.
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